1. Field of the Invention
The present invention relates to an organic nonlinear optical material to be used for optical information processing or optical communication. More particularly, the present invention relates to an organic compound having a large optical nonlinearity, excellent resistance to laser damage, and high durability.
2. Description of the Related Art
In the field of optoelectronics, research is being conducted to develop a material having an outstanding nonlinear optical effects never attained by the existing materials and the capability of a high speed response, to realize a nonlinear optical element having an extremely high performance.
The research has been concentrated mainly on inorganic materials, but a material satisfying the above requirements has not yet been found. Recently, an organic compound having a conjugated .pi. electronic system expected to have a large nonlinear optical effect and a high speed response in principle has, therefore, attracted much attention in the art.
With reference to organic materials having the second order optical nonlinearity, various types of compounds have been examined, and many disclosures of such materials have been made [(1) ACS, Symposium Series, 233 (1983) and (2) D. J. Williams Angew, Chem. Int. Ed. Engl. 23, p 690 (1984)].
For example, there can be mentioned benzene and pyridine derivatives such as N-(4-nitrophenyl)-L-prolinol (NPP) (Japanese Unexamined Patent Publication No. 59-21665 and U.S. Pat. No. 4,622,409), N-[5-(nitropyridyl-L-prolinol (PNP) and 2-acetylamino-4-nitro-N,N-dimethylaniline (DAN), stilbene derivatives such as 4-dimethylamino-4'-nitrostilbere (DANS) stilbazoliums such as 4'-dimethylamino-N-methyl-4-stilbazolium methosulfate (DMSM), and tenzylidene-aniline derivatives such as 4'-nitrobenzylidene[4-(N,N-dimethyl)aniline, 4'-nitrobenzylidene-4-methylaniline [Proceedings (Trudy) of the P. N. Leveder Physics Institute, Vol. 98 (1982), Basov, N. G. Editor (Consultants Bureau: New York, N.Y.), Shigorin, V. D. p 77: "Materials and Apparatus in Quantum Radio Physics")] and 4'-methylbenzylidene-4-ritroaniline.
It is believed that the optical nonlinearity of an organic compound having a conjugated .pi. electronic system is considered to be due to fluctuatiors of .pi. electrons responding to an applied optical electric field. There has been a guiding principle in designing organic nonlinear optical molecules, that is, electron-donating and electron-accepting substituents are introduced into conjugated .pi. electronic systems as in the above-mentioned known compounds.
In general, the crystal structure of an organic compound s determined by the structures of respective molecules. More specifically, hydrogen bonding at the time of molecular packing, and intermolecular cohesive forces such as the van der Waals interacticn and the dipole-dipole interaction determines the crystal structure in the balance of these forces.
If a highly electron-donating or electron-accepting substituent such as an amino group or a nitro group is introduced, the dipole moment possessed by the molecule is increfsed and the dipole-dipole interaction becomes strong upon formation of a crystal.
This strong dipole-dipole interaction among molecules according to the above-mentioned guiding principle of molecular design, that is, compounds having electron-donating and electron-accepting substituents in the conjugated .pi. electronic system, causes formation of a centrosymmetric crystal having a structure in which dipoles of two molecules negate each other.
In this centrosymmetric crystal, contrary to our wishes, the second order optical nonlinearity is not manifested.
There oan be seen a contrivance in a molecular design in the disclosed works that an optically active substituent or a substituent having a large hydrogen bond-forming capacity is introduced into the conjugated .pi. electronic system to destroy the centrosymmetry of the crystal, ghich hinders realization of the macroscopic optical nonlinearity in the crystalline state. There are examples of success, but those are limited to benzene derivatives and pyridine derivatives.
Typical instances are NPP, PNP, and DAN, but they fail to have such a large nonlinearity that enables a nonlinear optical element having a desired novel function to be realized.
This is thought to be because the molecular hyperpolarizability, which is an important factor in producing a molecular crystal having a sufficiently large nonlinearity for a practical use is small in benzene derivatives and in pyridine derivatives.
As the properties that should be possessed by nonlinear optical materials, there can be mentioned a large optical nonlinearity, a high speed optical response, a high transparency to laser beams, a high resistance to laser damage, phase matching characteristics, good crystal growth characteristics, high mechanical strength, and a high processability.
NPP and PNP are not satisfactory in terms of crystal growth characteristics in the melt phase or in the gas phase and have problems in that their durabilities are not sufficient because of their hygroscopicity or photodegradation properties. DAN is also defective in durability because of its hygroscopicity or sublimation pproperty.
A class of compounds such as stilbene derivatives and benzylidene aniline derivatives having an elongated conjugated .pi. electronic system in the main skeleton and an increased second order molecular hyperpolarizability, larger than 10 times that of the benzene derivatives such as p-nitroaniline, show only a macroscopic optical nonlinearity in the crystals smaller than those of the benzene derivatives since their dipole moments are accordingly increased and the crystals become centrosymmatic or even in the cases the crystals are noncentrosymmetric molecular alighments are not optimized.
Therefore, the introduction of a quaternary ammonium structure having a large steric hindrance is now under examingtion as a means of destroying the centrosymmetry of the crystal. A typical instance is DMSM manifesting a relatively large optical nonlinearity because of a large molecular hyperpolarizability and a noncentrosymmetric crystal structure. However, compounds of this type are unsatisfactory in terms of durability and processability because of the hygroscopicity and an easy structural change of crystal such as polymorphism is readily caused due to the quaternary ammonium structure.
As is apparent from the foregoing description, if only molecular decoration according to the conventional guiding principle of the molecular design is attempted, it is difficult to provide a nonlinear optical material exhibiting a large optical nonlinearity in the bulk state such as a crystal and satisfying the requirements indispensable for practical use.